28th International Cosmic Ray Conference

3569

Study of Cosmic Ray Short Term Variations Using
GRAPES-3 Muon Telescopes
India-Japan GRAPES Collaboration:
T. Nonaka,1 Y. Hayashi,1 Y. Ishida,1 N. Ito,1 S. Kawakami,1 A. Oshima,1S. Tamaki,1 H. Tanaka,1 T. Yoshikoshi1 K. Fujimoto,2 H. Kojima,2S.K. Gupta,3 Atul Jain,3 A.V. John,3 D.K. Mohanty,3 P.K. Mohanty,3S.D. Morris,3 K.C. Ravindran,3 K. Sivaprasad,3 B.V. Sreekantan,3S.C. Tonwar,3 and K. Viswanathan,3(1) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan(2) Nagoya Women’s University, Nagoya 467-8610, Japan(3) Tata Institute of Fundamental Research, Mumbai 400005, India

Abstract
We are operating a large area multi directional muon telescope as a partof the GRAPES-3 air shower experiment at Ooty (N 11.4, E 76.7 and 2200 maltitude). The muon detector has an area of 560 m2 and threshold energy of
1.0 GeV. Due to the large detector size, we are observing a very large numberof muons, 1.8X108 /hour. The design of the muon detector allow us to measuremuon intensity from 225 directions simultaneously. Therefore we are able to makea 2-dimensional map of cosmic ray intensity for protons of energy 65 GeV. Using
the response functions for the these 225 elements of the muon telescope,observed
intensity variation was analyzed especially around the period of disturbance. And
several Precursor variations which can be explained with Loss Cone model werefounded.
1.

GRAPES-3 Muon detector

GRAPES-3 multi directional muon telescope is consisted of proportionalcounters which have dimensions 10cm×10cm×600cm. Each detector module have4layers of 58 proportional counters. Each layer are placed in crossing conﬁguration. This conﬁguration enable us to measure arrival direction of penetrating
muon. Threshold energy for vertically penetrating muon is around 1GeV. We
are counting penetrating muon and categorizing into 15x15 direction by using hit
pattern marked by muon. There are 16 muon detectors. Total area of detector is560m2 . Fig. 1 shows the schematic view of module.[5]cpp. 3569–3572 2003
by Universal Academy Press, Inc.

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Fig. 1. Side view of GRAPES-3 Muon detector module. Each module are consistedof 4 layers of 58 proportional counters. 2m thick concrete were put on the top ofModule and 15cm concrete were mounted between each layers.

Fig. 2. Calculated response function of Vertical telescope[5]. Distribution of asymptotic direction were shown in top-left panel and corresponding average energy wereshown in bottom-right panel.Degree of color is corresponding to log E(GeV ) and haverange of 1GeV to 30TeV(no color mens no event).Other two panels shows projected
histograms of distribution of asymptotic direction along longitude and latitude.

2.

Calculation of response function

Muons were generated with monte-carlo simulation CORSIKA5.62(QGSJET-GHEISHA model)[1]. Geometry eﬀect and algorithm for angle determination were considered within detector simulation. We selected primaries which
created 4layer penetrating muon. To know asymptotic arrival direction,these primary particles were traced back in Earth’s magnetic ﬁeld up to 25×Earth radius[2]. Fig. 2 is the example of response function calculated by this method.
3.

Data analysis

Hourly counting rate were used for this analysis. To pickup event showing short term variation, we selected the days which shows standard deviation

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Fig. 3. Intensity variation observed in 9 categorized direction[5].Here intensity of
9-16 direction were summarized into NW,N,NE,W,V,E,SW,S and SE direction andplotted along time.Statistical error of each point is less than 0.03%.

grater than 0.4% from each module individually. To avoid false event with some
artiﬁcial cause,those short term variation has to be seen in all 16 modules. As aresult,around 30 candidate events were found in 2001 and 2002 data set. Some ofevent contain series of Forbush decreases.
From these data set we searched forevents which have correlation with Inter-planetary Magnetic Field. Fig. 4 is an example of observed intensity map and pitch angle contour map. Each cell(13×13)
shows intensity variation of central 169 directions. In this ﬁgure horizontal axis
are corresponding west to east direction and vertical axis are corresponding south
to north direction. The statistical error of each intensity data are 0.06 to 0.30%.
Pitch angle between each direction(13×13) and IMF were calculated using simulated response function at median rigidity. These value are also shown by contourline (15◦ /line) on the same ﬁgure. We examined this kind of map hour by hour.
4.

ResultSome of events were having good correlation between IMF direction. Weobserved dip in intensity map which seems to keep correlation for more than 24hrs
on 10th Apr 2001 and 11th Apr 2001. Fig. 3 is time proﬁles of the variation observedin 9 categorized direction. The variation which we are paying attention is around
UT04 10th Apr 2001 and UT03 11th Apr 2001. In Fig. 4 intensity variation of eachdirection around these period were plotted. 4maps those placed upside is variation
In Fig. 3 We can see Forbush
of 10th Apr bottom side is variation of 11thApr.
thdecrease on 11 Apr. It seems to be the result of ﬂares occurred UT0526 10th Aprand UT1534 9th Apr and associating CMEs. At this event geomagnetic storm
started around UT1500 11th Apr after an observation of inter planeterly shock at
UT1300 11th Apr.[4]3572

Fig. 4. Degree of intensity variation were shown by contour line drawn at each
0.3%.Range of color scale is -1.7% to +1.7%. Pitch angle between IMF fieldline(toward to sun) were drawn with dash-dot line at each 15◦ .

5.

Discussion and summary

Considering these situations,it is natural to understand variations on 10thApr 2001 and 11th Apr 2001 as a precursor phenomena of arrival of shock. The
deﬁcits before shock was enough large to discuss the structure. Loss cone model[3]can well describe these two dips with structural features. Deﬁcit on 10th Apr
seems to have a good correlation with IMF direction at median rigidity. Though
dip in 11th Apr didn’t show good correlation with IMF at median rigidity,thestructure of dips looks very similar to each other and the amplitude of 11th arequite large,nearly 1.8%. There is two possible explanations for this diﬀerence
between two loss cone. One is that on 11th ,distance from position of IMF structure
which is responsible for loss cone was so close that original direction of loss cone
didn’t align toward direction of IMF near Earth. Another is that the rigidity
spectrum of loss cone may softer than 10thApr. We think it is interesting tosearch best ﬁt loss cone parameters on 10th and 11th Apr and comparing eachother. We will continue to search consecutive precursors which shows this kindof behavior.
Acknowledgement :We thank all of GRAPES-3 member for supporting thisstudy. And We thank the ACE Science Center for providing the ACE data.
1. Heck D.et al. . 1998, FZKA 6019, Forschungszentrum Karlsruhe
2. A.Inoue.et al. . 1981,Institute of space and Aeronautical science report V1,79
3. K.Nagashima K.Fujimo.et al. . 1992, planet.space.sci.,Vol.40,No 8,1109
4. SEC Majordomo Archives . http://www.sel.noaa.gov
5. S.Kawakami.et al. . 2001, proc.27th ICRC(Humburg),V9,3473